The present invention relates to clock correction circuits and clock correction methods, and more particularly relates, for example, to a clock correction circuit and clock correction method used for a power meter and the like.
Recently, RTCs (Real Time Clocks) have been widely used in many information devices. One of examples on which an RTC is mounted is a power meter. The power meter calculates electricity charges and the like on the basis of the time period measured by the RTC. Generally, a tuning fork type crystal oscillator is used for generating the operation clock of the RTC, and the temperature characteristic of the frequency of the tuning fork type crystal oscillator is represented by a quadratic function that has a negative quadratic coefficient with about 25° C. as the center temperature. The temperature characteristic has, for example, a deviation about −150 ppm (parts per million) at the temperature −40° C. If the operation clock of the RTC has a deviation in its practical usage, the time period measured by the RTC has an error. Therefore, the correction of the frequency deviation of the operation clock is needed.
For example, in the power meter market, it is required that the accuracy of the clock generated by the RTC is within ±3 ppm or ±5 ppm. In the manufacturing process of power meters on which RTCs are mounted, it is inspected whether the clocks, the frequency errors of which have already been corrected, operate within desired accuracy or not. This inspection method is performed in a way that a clock (for example, 1 Hz) obtained by dividing a clock (for example, 32,768 kHz) generated by a crystal oscillator of an RTC is output from a terminal and that it is inspected whether the output clock satisfies the above-mentioned accuracy (±3 ppm, or ±5 ppm). Therefore, if 1 Hz clock is generated from the output clock of a crystal oscillator 32.768 kHz, the greatest accuracy of the generated clock is 30.5 ppm (1 Hz/32.768 kHz). The accuracy of this clock does not satisfy the accuracy required by the power meter market.
Japanese Unexamined Patent Application Publication No. 2000-315121 discloses an RTC circuit of a simple configuration that is capable of performing time correction with a high accuracy. The above RTC circuit includes an oscillator that generates a fundamental clock (for example, 32.768 kHz), and generates a frequency-divided signal obtained by dividing the fundamental clock. Further, the RTC circuit calculates the frequency errors of the fundamental clock generated by the oscillator with the use of a reference clock that is higher and more accurate than the fundamental clock. Successively, using the frequency-divided signal as a clock, an oscillator with a correction function in the RTC adds fixed values and the frequency errors, and outputs the MSB (Most Significant Bit, highest bit) of the addition result value as a correction clock. When the correction clock is generated, the frequency errors are accumulatively added, and when the accumulated value is reflected in the above MSB, the state of the correction clock is reversed, which corrects the frequency errors.